Conventionally, two gate voltage levels are used for operating (driving) power switches, i.e., low for OFF and high for ON. For many transistor devices that can act as power switches, a parasitic body diode exists which allows a PN-type Reverse Conduction Mode (RCM) when the device is turned off by gate control. Depending on the transistor device, also a usable reverse conduction mode may exist. This mode might be overlaid by the PN conduction mode.
For GaN transistor devices there generally is no body diode. However, GaN HEMTs have a reverse conduction mode. Moreover, the safe operating area (SOA) of a GaN HEMT depends on the drain-source voltage and the gate voltage of the device. This poses the following two problems.
Firstly, when the transistor device is in reverse conduction mode and the reverse VDS voltage is large, power dissipation losses will be high. This is clearly undesirable from a viewpoint of power conservation and efficiency.
Secondly, having a large VDS voltage and therefore a large device current when turning on the transistor device might violate the SOA. For example, circuits with long transition time for turning on the power device are likely to violate the SOA.